Conventionally, polyesters containing hydroxyl groups at the ends of the molecules have been used in the fields of paints, adhesives, polyurethanes and the like.
Known such polyesters include polyesters obtained from a polyalcohol such as ethylene glycol, diethylene glycol, propylene glycol, neopentyl glycol, 1,4-butanediol, 3-methyl-1,5-pentanediol, 1,6-hexanediol, trimethylolpropane or glycerin and a polybasic acid or an anhydrate thereof or an ester derivative thereof.
Among these polyesters, polyesters containing hydroxyl groups at the ends of the molecules obtained by an esterification reaction between an aliphatic dicarboxylic acid such as adipic acid and a dihydric or polyhydric alcohol are used in a wide range of applications such as elastomers, paints, adhesives, coatings and foams because they can be converted into polyurethanes by a reaction with an isocyanate compound having a functionality of 2 or more.
Similarly, polyesters containing carboxyl groups at the ends of the molecules can be converted into polyester polyamides having excellent heat resistance by a reaction with an isocyanate compound having a functionality of 2 or more.
In addition, polyesters obtained by an esterification reaction between a dihydric or polyhydric alcohol and a combination of polybasic acids, i.e., a combination of an aromatic dicarboxylic acid such as phthalic anhydride, isophthalic acid or terephthalic acid and an aliphatic dicarboxylic acid such as adipic acid are widely used in the fields of polyurethanes, paints, adhesives and the like. However, polyurethanes and the like obtained by using polyesters as polyol components have the disadvantage that they are susceptible to hydrolysis.
On the other hand, polyurethanes obtained by using polyethers are superior to polyurethanes obtained by using polyesters in hydrolysis resistance, but inferior in weather resistance as well as mechanical properties, oil resistance, and solvent resistance so that they are limited in their use. Further, polyurethanes obtained by using polycarbonates having excellent hydrolysis resistance improve the disadvantage described above, but the polyurethanes are insufficient in cold resistance and very expensive so that they are limited in their industrial use.
Therefore, there have been demands for polyesters that can provide polyurethanes retaining excellent properties such as flexibility, heat resistance (especially heat aging resistance), and mechanical properties and particularly having excellent hydrolysis resistance.
Known conventional polyester-based polyurethanes having relatively good hydrolysis resistance include polyurethanes obtained by using neopentyl glycol, or polyurethanes obtained by using 2-butyl-2-ethyl-1,3-propanediol (patent document 1), or polyurethanes obtained by using 2,4-dialkyl-1,5-pentanediol (patent document 2) or the like.
Alternatively, attempts have been made to improve hydrolysis resistance by using polyesters containing a branched-chain dicarboxylic acid unit having one methyl group side chain as starting materials for polyurethanes. For example, polyurethanes obtained by using polyesters derived from a dicarboxylic acid containing 3-methylpentanedioic acid (patent document 3) or 2-methyloctanedioic acid (patent document 4) as starting materials have been proposed.
These polyurethanes were improved in hydrolysis resistance as compared with the previous polyurethanes, but still insufficient.
On the other hand, patent document 5 describes polyesters for use as starting materials of polyurethanes. Specifically, Example 4 of patent document 5 describes that a polyester obtained by reacting 3,3′-oxybis(2-ethyl-2-butyl-1-propanol) and terephthalic acid in an equimolar ratio shows stability against thermal degradation. Further, it describes that such a polyester is useful as an extenders for polyurethanes.
In addition, patent document 6 and patent document 7 disclose processes for continuously preparing polyester resins containing an aromatic dicarboxylic acid and an aliphatic diol as main components.